Macrokinetics of polyaniline based electrode: effects of porous structure, microkinetics, diffusion, and electrical double layer

Abstract

The standard porosimetry method (SPM) was used to study in situ real porous structures of poly(aniline) (PAn) swollen in electrolytes containing γ-butyrolactone (GBL). The effects of swelling, the degree of doping, the nature of counterions, and type the of polymerization (chemical or electrochemical) on porous PAn structure were studied. All the PAn forms impregnated with GBL had high porosities of several dozen per cent, high specific surface areas of 80–400 m 2 cm −3, pore radii of 1–100 nm, and average fibril radii of 2–10 nm. Electrochemically synthesized PAn showed better electrochemical characteristics than chemically synthesized PAn. The model of the intercalated porous electrode was used to characterize the discharge curves. Comparison of calculated and experimental curves enabled determination of essential microscopic characteristics: specific exchange current density ( i 0) of real PAn/electrolyte interphase doping–dedoping, specific capacity ( C S) of electrical double layer (EDL) of this interface, and diffusion coefficient ( D) of counterions in the polymer. These characteristics depend upon the nature of counterions and the method of polymerization (chemical or electrochemical). Small D values of 8×10 −19–1×10 −17 cm 2 s −1 result from the solid-phase nature of diffusion; i 0=1×10 −15–1×10 −12 A cm −2, and C S=0.6–5.0 μF cm −2. Very low D, i 0 and C S are compensated by very high S resulting in better electrochemical characteristics. Thin electrodes (<1 mm) showed very small ohmic losses, while the rate of electrochemical doping–dedoping was usually limited by solid-phase diffusion and discharge. The effects of structure, mode, and microscopic parameters on discharge curves were analyzed.